Avestan

Avestan is the language of ancient Persia, surviving only in the sacred Zoroastrian scriptures known as the Zend Avesta. In the past the language was known as 'Zend', but this was later considered a misuse of this word. The oldest form of Avestan, spoken by Zoroaster himself is found in the Gathas[?]. This is a dialect variant of Vedic, the oldest known form of the Sanskrit language.

Western study of Vedic and Avestan at the end of the 18th century was responsible for the discovery of the Indo-European language group, and thus to the creation of modern linguistics and philology.